In common mobile communication environment, signals communicated between the base station and the mobile terminal are transmitted along several paths between the receiver and the transmitter. Due to difference in propagation paths, the same signal may arrive at the receiver via different paths with different propagation delays and DOAs (directional angle of arrival), thus multi-path interference and signal fading are caused.
By taking full advantage of the space property of signals, array antenna techniques can reduce multi-path interference and signal deterioration effectively, improve system capacity and QoS markedly, and thus won wide applications in real life.
For the array antenna, beamforming is a basic function. That is, the array antenna can perform operations like delaying, weighting and combining to the signals received by the antenna elements, to form antenna beams whose major lobe aims at the direction of the user signals and null at that of the interference signals, so as to suppress the interference. Thereby the beams formed by the array antenna have significant effect on system performance.
FIG. 1 is a schematic diagram illustrating a one-dimension linear array antenna comprising M elements. As shown in FIG. 1, θ is the incident signal elevation, d is the space between elements (geometrical aperture), and all elements are assumed to have the same space. The half-power beam width of the array antenna, θ0.5, is approximately:
                              θ          0.5                ≈                              50.8            ·            c                                M            ·            d            ·            f                                              (        1        )            wherein M is the number of antenna elements, f is the carrier frequency of the signals, and c is velocity of light which equals to 3×108 m/s.
The geometrical aperture d, and the number of antenna elements M, are generally constant, which means the length of the antenna array, M·d, is constant too.
As it can be seen from equation (1), if the length of the antenna array M·d, is fixed, the antenna can form beams with different width when receiving signals with different frequencies. The higher the frequency is, the narrower the beam width is. Researches indicate that the beam width is in inverse ratio to the signal frequency. When broadband signals are received not in the direction to which the beam heads, the beam width of the antenna is relatively narrow for HF (high frequency) signals, so part of HF signals will fall to the null of the antenna pattern, thus the energy of these signals will be lost by the beam output. Accordingly, the output of the antenna is distorted.
To deal with the above-mentioned problem of antenna output distortion, the present invention provides a beamforming method based on broadband antenna.